Many different hydrated zinc borates are known and several find commercial application as fire retardants and smoke suppressants for various polymers. Hydrated zinc borates are also used as anti-corrosive pigments for coatings and have demonstrated fungistatic and bacteriostatic properties which find many applications.
The known hydrated zinc borates include ZnO.B.sub.2 O.sub.3.H.sub.2 O, 2ZnO.3B.sub.2 O.sub.3.5H.sub.2 O, 2ZnO.3B.sub.2 O.sub.3.7H.sub.2 O, 3ZnO.5B.sub.2 O.sub.3.14H.sub.2 O (sometimes designated 2ZnO.3B.sub.2 O.sub.3.9H.sub.2 O), ZnO.B.sub.2 O.sub.3.2H.sub.2 O, ZnO.5B.sub.2 O.sub.3.4.5H.sub.2 O, 2ZnO.3B.sub.2 O.sub.3.3H.sub.2 O, and 6ZnO.5B.sub.2 O.sub.3.3H.sub.2 O. See Supplement to Mellor's Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. V, Part A, Pages 577-578, Longman Group Ltd (1980). See also pages 572-6 for a review of anhydrous zinc borates. Several of these zinc borate hydrates have commercial importance, especially as fire retardants and smoke suppressants for polymers, rubbers and coatings. The relatively high dehydration temperature (about 290.degree. C.) of 2ZnO.3B.sub.2 O.sub.3.3.5H.sub.2 O offers a significant advantage over other commercially available hydrated zinc borates since elevated temperatures are required for compounding many plastics and rubbers. However, some of the recently developed engineering plastics require processing at even higher temperatures in the range of about 300.degree.-400.degree. C., and it is desirable to have a zinc borate which has an even higher dehydration temperature.